1. Field of the Invention
The present invention relates to a fuel cell formed by stacking a membrane electrode assembly and a separator in a stacking direction. The membrane electrode assembly includes a pair of electrodes, and an electrolyte membrane interposed between the electrodes. A reactant gas flow field for supplying a reactant gas along an electrode surface is formed in the separator.
2. Description of the Related Art
For example, a solid polymer electrolyte fuel cell employs a membrane electrode assembly (MEA) which includes an anode, a cathode, and an electrolyte membrane interposed between the anode and the cathode. The electrolyte membrane is a solid polymer ion exchange membrane. The membrane electrode assembly and separators sandwiching the membrane electrode assembly make up a unit of a power generation cell (unit cell). In use, generally, several tens to hundreds of unit cells are stacked together to form a fuel cell stack mounted in a vehicle.
In the fuel cell, so called internal manifolds are often adopted for supplying a fuel gas and an oxygen-containing gas as reactant gases to the anode and the cathode of each of the stacked power generation cells. The internal manifold includes a reactant gas supply passage and a reactant gas discharge passage extending through the power generation cells in the stacking direction. The reactant gas supply passage and the reactant gas discharge passage are connected respectively to an inlet and an outlet of a reactant gas flow field for supplying the reactant gas along an electrode surface.
In this regard, openings of the reactant gas supply passage and the reactant gas discharge passage have relatively small sizes. Therefore, in order to allow the reactant gas to flow smoothly, buffers for distributing the reactant gas are required at positions adjacent to the reactant gas supply passage and the reactant gas discharge passage. For example, a solid oxide fuel cell disclosed in Japanese Laid-Open Patent Publication No. 06-140056 (hereinafter referred to as the conventional technique) includes a separator 1 as shown in FIG. 16.
Supply/discharge holes 2a as passages of one of the reactant gases and supply/discharge holes 2b as passages of the other of the reactant gases are provided respectively at diagonally opposite positions, i.e., at four corners of the separator 1. Ridges and grooves are provided alternately on a surface 1a of the separator 1 to form a reactant gas flow field 3a. Likewise, a reactant gas flow field 3b is formed on a surface 1b of the separator 1.
On the surface 1a, the supply/discharge holes 2a and the reactant gas flow field 3a are connected through gas distribution channels (buffers) 4a, and a plurality of current collectors 5 are provided in the gas distribution channels 4a. On the surface 1b, gas distribution channels 4b connecting the supply/discharge holes 2b and the reactant gas flow field 3b are formed, and a plurality of current collectors 5 are provided in the gas distribution channels 4b. 